Aluminum base part and manufacturing method

ABSTRACT

An aluminum base part is composed of a forging cast product produced by a forging cast process and a nickel chromium plating layer formed on the forging cast product. The nickel chromium plating layer is composed of a semibright nickel plating layer formed on the forging cast product and having a thickness of 5 to 10 μm, a bright nickel plating layer  33  formed on the semibright nickel plating layer  31  and having a thickness of 5 to 10 μm, and a chromium plating layer  37  formed on the bright nickel plating layer  33  and having a thickness of 0.5 to 3 μm. The present invention adopts a forging cast product rather than a cast product. The forging cast product has a smooth surface, so that a plating layer can be directly formed on the surface of the forging cast product without forming an undercoat resin layer.

TECHNICAL FIELD

The present invention relates to a manufacturing technique for analuminum base part.

BACKGROUND OF THE INVENTION

An aluminum base part is sometimes used for a handle holder in amotorcycle or a door knob and emblem in a passenger car. In this case,the aluminum base part becomes an exterior part exposed to a person'ssight, and a good appearance of the aluminum base part is thereforedesired.

Various techniques for improving the appearance of an aluminum base parthave been proposed (see PCT Patent Publication No. WO 00143153 pamphlet(page 22) and Japanese Patent Laid-Open No. 2004-17738, for example).

SUMMARY OF THE INVENTION

The invention disclosed in PCT Patent Publication No. WO 00143153discloses a surface treating method for a light metal casting includinga casting step for applying a casting pressure of greater than about 50megapascals from an injection plunger to a molten metal of light metalmaterial supplied into a die to thereby form a casting so that pin holesgenerated on its casting surface are suppressed to meet predeterminedconditions; a polishing step for polishing the casting surface so thatthe surface roughness of a polished surface becomes a predeterminedvalue or less; a coating step for forming a first resin coating layer onthe polished surface; and a plating step for forming a metal or metalcompound layer on the surface of the first resin coating layer by dryplating.

According to this method, the size and number of pin holes aresufficiently reduced by squeeze casting. Further, barrel polishing isnext performed to reduce the surface roughness. Thereafter, resin iscoated, and plating is next performed. As a result, a light metal castproduct having a bright design surface can be provided.

However, the coating step for coating the resin is indispensable, thusthe number of man-hours increases, causing an increase in productioncost.

Japanese Patent Laid-Open No. 2004-17738 discloses a surface brightenedlight alloy wheel for an automobile including a light alloy wheel bodyhaving a minutely rough surface, a resin coating formed on the surfaceof the light alloy wheel body, a titanium alloy thin film formed on theresin coating and having a chromium plating appearance, and atransparent resin protective layer formed on the titanium alloy thinfilm. Thus, the resin coating is formed on the surface of the lightalloy wheel body. Also in this technique, a coating step for forming theresin coating is indispensable, thus the number of man-hours increases,causing an increase in production cost.

Accordingly, if the resin coating step can be eliminated, the number ofman-hours can be reduced, thus reducing a production cost.

It is accordingly an object of the present invention to provide atechnique which can improve the appearance of an aluminum base partwithout applying a resin as an undercoat.

There is provided an aluminum base part adapted to be provided on theexterior surface of a vehicle, including a forging cast product producedby a forging cast process and a nickel chromium plating layer formed onthe forging cast product.

Further, the nickel chromium plating layer includes a semibright nickelplating layer formed on the forging cast product and having a thicknessof 5 to 10 μm, a bright nickel plating layer formed on the semibrightnickel plating layer and having a thickness of 5 to 10 μm, and achromium plating layer formed on the bright nickel plating layer andhaving a thickness of 0.5 to 3 μm.

There is provided a manufacturing method for an aluminum base partadapted to be provided on the exterior surface of a vehicle, includingpreparing a pressing device including a die having upper and loweropenings, a lower punch for closing the lower opening of the die, anupper punch opposed to the lower punch and adapted to be inserted intothe die from its upper opening, a first cylinder unit for raising andlowering the upper punch, a partial punch formed as a divisional part ofthe upper punch, and a second cylinder unit for raising and lowering thepartial punch; supplying an aluminum alloy molten metal into the die towhich the lower punch is fitted; a primary pressing step performed bycollectively inserting the upper punch and the partial punch into thedie from the upper opening of the die to compress the molten metal; asecondary pressing step performed by increasing a pressing pressure ofthe upper punch but not the partial punch, generated by the firstcylinder unit to further insert the upper punch into the die, therebyfurther compressing the molten metal; releasing a forging cast productfrom the die after solidification of the molten metal; and a plating theforging cast product with a nickel plating layer and a chromium platinglayer.

The manufacturing method further includes polishing the surface of theforging cast product between the forming step and the plating step sothat the average surface roughness of the forging cast product becomes 6μm or less.

According to the invention, a forging cast product rather than a castproduct is adopted. The forging cast product has a smooth surface, sothat a plating layer can be directly formed on the surface of theforging cast product without forming an undercoat resin layer.Accordingly, the appearance of the aluminum base part can be improvedwithout applying a resin as an undercoat, and the number of man-hourscan therefore be reduced.

According to the invention, a semibright nickel plating layer having athickness of 5 to 10 μm is first formed on the forging cast product, abright nickel plating layer having a thickness of 5 to 10 μm is nextformed on the semibright nickel plating layer, and a chromium platinglayer having a thickness of 0.5 to 3 μm is finally formed on the brightnickel plating layer.

Thus, the semibright nickel plating layer having a thickness of 5 μm ormore and the bright nickel plating layer having a thickness of 5 μm ormore are formed as an undercoat layer, so that a sufficient corrosionresistance can be maintained. Furthermore, the maximum thickness of eachnickel plating layer is set to 10 μm, so that the consumption of nickelcan be suppressed.

The two nickel plating layers having a total thickness of 10 μm at aminimum are formed as an undercoat on the forging cast product.Accordingly, the thickness of the chromium plating layer as a top coatcan be sufficiently reduced, and although it is reduced to 0.5 μm, ametallic luster can be produced to thereby maintain a good appearance.Further, a maximum thickness of the chromium plating layer is 3 μm, andthus the consumption of chromium can be suppressed.

According to the invention, the forging cast product is produced by atwo-stage process composed of the primary pressing step and thesecondary pressing step. By strongly compressing the molten metal, thedensity of the forging cast product can be increased. In the secondarypressing step, only the partial punch, which is independent of the upperpunch, is allowed to be raised so that an excess amount of molten metalcan escape toward the partial punch. Accordingly, even when the amountof molten metal supplied into the die is varied, this variation insupply amount can be absorbed by the amount of the rise of the partialpunch. Accordingly, the finished shape of the forging cast product canbe improved.

According to the invention, the forging cast product obtained by theforming step is polished so that the average surface roughness of theforging cast product is 6 μm or less. By setting the average surfaceroughness to be 6 μm or less, the appearance of the aluminum base partcan be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention will become apparent in the followingdescription taken in conjunction with the drawings, wherein:

FIG. 1 is a schematic view showing the principle of a pressing deviceusable in the present invention;

FIGS. 2( a) to 2(c) are schematic views for illustrating a forging castprocess adopted in the present invention;

FIGS. 3( a) to 3(f) are schematic views for illustrating a forming stepto a plating step in the present invention;

FIG. 4( a) is a sectional view of an aluminum base part according to thepresent invention, and FIG. 4( b) is an enlarged view of an encircledportion b in FIG. 4( a);

FIG. 5 is a graph showing the correlation between the ratio between thelayer thicknesses and the corrosion resistance;

FIG. 6 is a graph showing the correlation between the layer thicknessesand the appearance; and

FIG. 7 is a graph showing the correlation between the surface roughnessof a forging cast product and the appearance.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention will now be describedwith reference to the attached drawings.

FIG. 1 is a schematic view showing the principle of a pressing device 10used in this preferred embodiment. The pressing device 10 is composed ofa cylindrical die 11 having upper and lower openings, a lower punch 12for closing the lower opening of the die 11, an upper punch 13 opposedto the lower punch 12, a first cylinder unit 14 for raising and loweringthe upper punch 13, a partial punch 15 formed as a divisional part ofthe upper punch 13, a second cylinder unit 16 for raising and loweringthe partial punch 15, a support 17 for commonly supporting the firstcylinder unit 14 and the second cylinder unit 16, a guide 18 for guidingthe support 17 so as to prevent its oscillation, a third cylinder unit19 for raising and lowering the support 17, a fourth cylinder unit 21for raising and lowering the lower punch 12, and a base 22 forsupporting the die 11.

The upper punch 13 has a hollow cylindrical shape, and the firstcylinder unit 14 for raising and lowering the upper punch 13 also has ahollow cylindrical shape. The partial punch 15 is vertically movablyinserted in the central hollow portion of the upper punch 13, and thesecond cylinder unit 16 for raising and lowering the partial punch 15 isinserted in the central hollow portion of the first cylinder unit 14.

There will now be described a forging cast process performed by usingthe pressing device 10 mentioned above.

FIGS. 2( a), 2(b), and 2(c) are schematic views for illustrating theforging cast process according to the present invention. As shown inFIG. 2( a), a predetermined amount of molten metal 24 of aluminum alloyis supplied into the die 11 to which the lower punch 12 is fitted (asshown by an arrow (1)). Examples of the aluminum alloy include JIS A6000family, JIS ADC12, JIS AC4CH, and equivalent. However, the kind of thealuminum alloy as the molten metal 24 is not especially limited.

The third cylinder unit 19 is next operated to collectively lower thesupport 17, the first cylinder unit 14, the second cylinder unit 16, theupper punch 13, and the partial punch 15.

As shown by arrows (2) in FIG. 2( b), the upper punch 13 and the partialpunch 15 are inserted into the die 11 from the upper opening of the die11 to compress the molten metal 24. At this time, the pressing pressureof the upper punch 13 (the pressure applied to unit area of the surfaceof the molten metal 24, the same applying to the following description)is set equal to the pressing pressure of the partial punch 15. Since thepressing pressure of the upper punch 13 is equal to that of the partialpunch 15, the upper punch 13 and the partial punch 15 are loweredtogether. This lowering operation will be hereinafter referred to as aprimary pressing step.

By compressing (pressurizing) the molten metal 24 in the primarypressing step, a minute amount of gases (especially, H₂ gas) containedin the molten metal 24 can be expelled. That is, degassing of the moltenmetal 24 can be attained.

The hydraulic pressure of the first cylinder unit 14 is next increasedwhile the hydraulic pressure of the second cylinder unit 16 is keptunchanged. Accordingly, as shown in FIG. 2( c), the upper punch 13 islowered as shown by arrows (3). As a result, the molten metal 24 isstrongly compressed to escape and push up the partial punch 15.

That is, the partial punch 15 is raised as shown by an arrow (4). Whenthe pressing pressure of the upper punch 13 and the pressing pressure ofthe partial punch 15 are balanced, the movement of the upper punch 13and the partial punch 15 is stopped. In this condition, solidificationof the molten metal 24 is performed. This operation shown in FIG. 2( c)will be hereinafter referred to as a secondary pressing step.

The partial punch 15 serves as a buffer. Accordingly, even when theamount of the molten metal 24 supplied in the step of FIG. 2( a) isvaried, the molten metal 24 can be pressurized without any trouble inthe step of FIG. 2( c).

After the molten metal 24 is solidified in the step of FIG. 2( c), theupper punch 13 and the partial punch 15 are raised. Thereafter, thefourth cylinder unit 21 is operated to raise the lower punch 12.Accordingly, the forging cast product can be released from the die 11.

As described above, the molten metal 24 is pressurized by the upperpunch 13 and the partial punch 15 in the step of FIG. 2( b). That is,the first stage of the forging cast process is performed as shown inFIG. 2( b). In addition, the molten metal 24 is forged by the loweringupper punch 13 and the rising partial punch 15, so that the molten metal24 is pressurized more strongly. Thus, the second stage of the forgingcast process is performed as shown in FIG. 2( c). By stronglypressurizing the molten metal 24, the density of the molten metal 24 canbe increased and the generation of pin holes on the surface (castingsurface) of the molten metal 24 solidified can be greatly reduced.

That is, by the two-stage forging cast process composed of the primarypressing step and the secondary pressing step, degassing of the moltenmetal 24 can be sufficiently performed to thereby obtain the forgingcast product having a smooth casting surface.

The subsequent steps to be applied to the forging cast product obtainedabove will now be described with reference to FIGS. 3( a) to 3(f).

FIGS. 3( a) to 3(f) are schematic views for illustrating a forming stepto a plating step in the present invention. As shown in FIG. 3( a), anundesired portion 25 is removed to obtain a formed forging cast product26 as shown in FIG. 3( b). This forging cast product 26 may be directlysubjected to plating. However, as shown in FIG. 3( c), the forging castproduct 26 is preferably polished by using a buff 27 or a polishing beltprior to plating. By this polishing step, the average roughness Ra(arithmetic mean roughness defined by JIS B0601) of the surface of theforging cast product 26 is set to 6 μm or less. In other words, theforging cast product 26 is polished so that the maximum value of theaverage roughness Ra is 6 μm.

As shown in FIG. 3( d), a polished forging cast product 28 obtained bythe step of FIG. 3( c) (or the formed forging cast product 26) is nextset in a semibright nickel electroplating device 29 to form an undercoatlayer. The thickness of the undercoat layer may be determined byincreasing or decreasing the period of time for conducting.

As shown in FIG. 3( e), the forging cast product 26 or 28 coated with asemibright nickel plating layer 31 as the undercoat layer is next set ina bright nickel electroplating device 32 to form an intermediate coatlayer. The thickness of the intermediate coat layer may be determined byincreasing or decreasing the period of time for conducting.

As shown in FIG. 3( f), the forging cast product 26 or 28 coated with abright nickel plating layer 33 as the intermediate coat layer is nextset in a chromium electroplating device 34 to form a top coat layer. Thethickness of the top coat layer may be determined by increasing ordecreasing a conducting period of time.

FIG. 4( a) is a sectional view of an aluminum base part 35 according tothe present invention. For example, the aluminum base part 35 is ahandle holder for fixing a steering handle of a motorcycle. As shown inFIG. 4( a), the aluminum base part 35 is composed of the forging castproduct 28 or 26 produced by the forging cast process mentioned aboveand a nickel chromium plating layer 36 formed on the forging castproduct 28 or 26.

FIG. 4( b) is an enlarged view of an encircled portion b in FIG. 4( a).As shown in FIG. 4( b), the nickel chromium plating layer 36 is composedof the semibright nickel plating layer 31 formed on the surface of theforging cast product 28 or 26 and having a thickness of 5 to 10 μm, thebright nickel plating layer 33 formed on the semibright nickel platinglayer 31 and having a thickness of 5 to 10 μm, and a chromium platinglayer 37 formed on the bright nickel plating layer 33 and having athickness of 0.5 to 3 μm.

Thus, the nickel plating layers 31 and 33 are formed under the chromiumplating layer 37 in the nickel chromium plating layer 36. The chromiumplating layer 37 as the top coat layer functions to improve theappearance, and the nickel plating layers 31 and 33 as the undercoat andintermediate coat layers function to maintain a sufficient corrosionresistance. The thickness of the chromium plating layer 37, which is notrequired to have corrosion resistance, may be small, but the thicknessesof the nickel plating layers 31 and 33, which are required to havecorrosion resistance, must be determined carefully.

The thicknesses of the nickel plating layers 31 and 33 will now bediscussed.

FIG. 5 is a graph showing the correlation between the ratio between thelayer thicknesses and the corrosion resistance. The ratio between thelayer thicknesses is defined as the ratio of the thickness of thesemibright nickel plating layer 31 to the thickness of the bright nickelplating layer 33. The corrosion resistance was examined with this ratiobeing changed. As the result of this examination, it was found that whenthe ratio is 1.0, the corrosion resistance is highest. Accordingly, thethickness of the semibright nickel plating layer 31 was equal to oralmost equal to the thickness of the bright nickel plating layer 33.

FIG. 6 is a graph showing the correlation between the layer thicknessesand the appearance. The ratio between the layer thicknesses was fixed at1.0, and the thickness of the semibright nickel plating layer 31 and thethickness of the bright nickel plating layer 33 were changed to examinethe appearance. As the result of this examination, it was found that thelarger the thicknesses of these layers 31 and 33, the better theappearance.

Further, it was determined that when the thickness of the semibrightnickel plating layer 31 is 5 μm or more and the thickness of the brightnickel plating layer 33 is 5 μm or more, the appearance can exceed anacceptable line.

However, when the thickness of the semibright nickel plating layer 31exceeds 10 μm and the thickness of the bright nickel plating layer 33exceeds 10 μm, the curve showing this correlation becomes flat, so thatthe appearance was not remarkably improved.

Accordingly, it is preferable that the thickness of the semibrightnickel plating layer 31 is 5 to 10 μm and the thickness of the brightnickel plating layer 33 is also 5 to 10 μm.

The polishing performed in the step of FIG. 3( c) will now be examined.

FIG. 7 is a graph showing the correlation between the surface roughnessof the forging cast product and the appearance. A forging cast producthaving an average surface roughness Ra of 1 to 7 μm was prepared and thenickel chromium plating layer 36 was formed on this forging castproduct, wherein the thickness of the semibright nickel plating layer 31to 5 μm, the thickness of the bright nickel plating layer 33 was 5 μm,and the chromium plating layer 37 was 0.5 μm. By using these sampleshaving different values for the average surface roughness Ra, theappearance was evaluated. As the result of this evaluation, it was foundthat the smaller the average surface roughness Ra, the better theappearance. Further, it was determined that when the average surfaceroughness Ra is 6.0 μm or less, the appearance can exceed an acceptableline.

Accordingly, when the average surface roughness Ra of the forging castproduct (reference numeral 26 shown in FIG. 3( b)) is 6.0 μm or less,polishing is not performed, but the plating is directly applied to theforging cast product 26. However, when the average surface roughness Raof the forging cast product 26 is greater than 6.0 μm, polishing ispreferably performed to reduce the average surface roughness Ra to 6.0μm or less.

While the nickel chromium plating layer 36 is composed of the two nickelplating layers 31 and 33 and the single chromium plating layer 37 inthis preferred embodiment, one nickel plating layer or three or morenickel plating layers may be used in place of the two nickel platinglayers 31 and 33. Thus, the number of nickel plating layers isarbitrary.

Further, while the forging cast process preferably includes a two-stagepressing process composed of the primary pressing step and the secondarypressing step in this preferred embodiment, the forging cast processaccording to the present invention may include a single-stage pressingprocess composed of only the primary pressing step or only the secondarypressing step. Further, the forging cast process may include a three ormore-stage pressing process.

The present invention is applicable to an aluminum base part as anexterior part of a vehicle.

Although a specific form of embodiment of the instant invention has beendescribed above and illustrated in the accompanying drawings in order tobe more clearly understood, the above description is made by way ofexample and not as a limitation to the scope of the instant invention.It is contemplated that various modifications apparent to one ofordinary skill in the art could be made without departing from the scopeof the invention which is to be determined by the following claims.

1. An aluminum base part, comprising: a forging cast product made ofaluminum and produced by a forging cast process; and a nickel chromiumplating layer formed on said forging cast product.
 2. The aluminum basepart according to claim 1, wherein said nickel chromium plating layercomprises: a semibright nickel plating layer formed on said forging castproduct and having a thickness of 5 to 10 μm, a bright nickel platinglayer formed on said semibright nickel plating layer and having athickness of 5 to 10 μm, and a chromium plating layer formed on saidbright nickel plating layer and having a thickness of 0.5 to 3 μm. 3.The aluminum base part according to claim 2, wherein the thickness ofsaid semibright nickel plating layer is approximately equal to thethickness of said bright nickel plating layer.
 4. The aluminum base partaccording to claim 1, wherein said forging cast product has a surfaceroughness Ra of 6.0 μm or less.
 5. The aluminum base part according toclaim 2, wherein said forging cast product has a surface roughness Ra of6.0 μm or less.
 6. The aluminum base part according to claim 3, whereinsaid forging cast product has a surface roughness Ra of 6.0 μm or less.7. A method of manufacturing an aluminum base part, comprising thestep(s) of: preparing a pressing device including: a die having upperand lower openings, a lower punch for closing the lower opening of saiddie, an upper punch opposed to said lower punch and adapted to beinserted into said die from the upper opening of said die, said upperpunch including a partial punch formed as a divisional part of saidupper punch, and a first cylinder unit for raising and lowering saidupper punch, supplying an aluminum alloy molten metal into said die towhich said lower punch is fitted; collectively inserting said upperpunch and said partial punch into said die from the upper opening of thedie to compress said molten metal; increasing a pressing pressure ofsaid upper punch but not increasing a pressing pressure of said partialpunch, to further insert said upper punch into said die and furthercompress said molten metal, said pressing pressure of said upper punchbeing generated by said first cylinder unit; releasing a forging castproduct from said die after solidification of said molten metal; nickelplating said forging cast product with a nickel plating layer to form anickel plated forging cast product, and chromium plating said nickelplated forging cast product with a chromium plating layer.
 8. The methodof claim 7, further comprising the step of: polishing a surface of saidforging cast product between said releasing step and said plating stepuntil an average surface roughness of said forging cast product is 6 μmor less.
 9. The method of claim 7, wherein said nickel plating stepincludes the steps of: plating said forging cast product with asemibright nickel electroplating layer to form a semibright nickelelectroplated forging cast product, and then plating said semibrightnickel electroplated forging cast product with a bright nickelelectroplating layer.
 10. The method of claim 8, wherein said nickelplating step includes the steps of: plating said forging cast productwith a semibright nickel electroplating layer to form a semibrightnickel electroplated forging cast product, and then plating saidsemibright nickel electroplated forging cast product with a brightnickel electroplating layer.